Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Current Driving

In succession to the previous article, ”Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Voltage Driving”, here the use of a BTL amplifier for linear current driving is explained.

Driving of Brushed DC Motors Using BTL Amplifier Circuits: Linear Current Driving

The following is a BTL amp circuit that performs linear current driving of a brushed DC motor.

A resistor R_s for current detection is inserted between the motor and the output OUT1, so that the current flowing to the motor is detected as a voltage, which is used for negative feedback to the input-stage amplifier. Through this feedback, the input-stage inverting amplifier performs feedback control of the output voltages of the two output-stage power amplifiers. The output-stage amplifiers are differential amps; the output M0 of the input-stage amp is received by the inverting input of the OUT1 amp, and is received by the non-inverting input of the OUT2 amp, so that OUT2 is opposite in phase from OUT1. Overall, the circuit can be considered to be a single-ended input, differential output (normal phase/inverted phase) amplifier.

The relationship between the input voltage and the output current is as follows. The output current (motor driving current) is fed back to the inverting terminal of the input-stage inverting amplifier as the output V_so of the differential amplifier that detects and amplifies the voltage difference (voltage drop) across the current detection resistor R_s. V_so can be found using the equation below. As is clear from the connections of R_s, V_so is a voltage that is inverted with respect to the current direction.

V_so＝－R_s・I_o・(R6/R5)＋V_ref

Further, if the voltage applied to the input terminal IN1 is VIN1, then upon setting the input-stage amp gain to be infinitely high, the voltage at the inverting input terminal becomes V_ref.

From the principle of operation of an inverting amplifier, control is executed such that the sum of the currents at the inverting input terminal of the input-stage inverting amplifier is 0 A. That is, the following equation obtains.

 which, modified, becomes 

R_s(R6/R5) is a fixed value, and so the output current I_o can be set to a desired value by adjusting VIN1.

Current driving has the feature that, when the current is less than the current setting, the motor is driven at maximum voltage, so that the time until the current setting is reached is short, and the motor response is fast. Moreover, in this article an example of linear driving was presented, but similarly for PWM output, current driving is possible by detecting the current and executing negative-feedback control.